Rectifier timing circuit



Patented Sept. 5, 1944 UNI-TED STATES PATENT oFFlcE 2,357,652 f nnorirmn TIMING omcmr vRobert L. Haynes, Indianapolis, Ind., assignor to Radio Corporation'of America, a corporation of Delaware Application May 25, 1942,'seria1N0. 444,463 9 claims. (ci. 11e-100.3)

This invention relates to current control systems and particularly to rectifier-filter combinations for translating alternating currents into direct currents having predetermined voltage variations with respect to the alternating cur-- rent variations.

In the ,art of sound recording wherein alterhating current signals are recorded on lm, noise reduction is generally applied. In the application of noise reduction it is necessary to rectify the signal currents, the resulting direct currents being impressed upon the noise reduction portion of the recorder which may be either the light -'modulator itselfor a shutter or shutters to control the average amount of light as the signal varies in amplitude. In order to impart the proper action to the noise reduction element duryingyincreases and decreases in the signal, timing of the noise reduction element must be such as to avoid flutter at low frequencies or during low frequency modulations and at the sametime close with suflicient rapidity so as to provide the maximum amount of noise reduction.

mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawings forming a part hereof, in which:

vFig. 1 is a schematic circuit diagram of a simple embodiment of the invention; Fig. 2 is a graph illustrating the operation of v the circuit shown in Fig. 1;

Fig. 3 is a schematic circuit diagram of another embodiment ofthe invention;

Figs. 4, 5, 6 and 7 are graphs showing the action of the circuit of Fig. 3;

Fig. 8 is a schematic circuit diagram of another embodiment of the invention employing a ,double rectifier arrangement;

Figs. 9, 10, l1 and 12 are graphs showing the action of the circuit of Fig. 8; Y i

Fig. 13 is a schematic circuit diagram showing another modification of the invention;

Figs. 14,- 15, 16 and 17 are graphs showing the opera-tion of the circuit of Fig. 13.

I source of potential 6l in series.

The present invention is directed to a noise reduction system which provides a particularly desirable closing action, the principal object of the invention being to facilitate andv improve the recording of noiseless sound records.

Another object of the invention is to control the noise reduction action of a sound recording system in a particular predetermined manner.

A further object of the invention is to provide the noise reduction elements of a sound recording system with Aa slow response immediately after the end of or decrease in a sound signal followed by a rapid response.

A further object of the invention is to provide a noise reduction circuit having timing elements which produce a slow closing action over a predetermined time period followed by a more rapid closing action. v

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the. 55

Referring now to Fig. 1, which shows the general principal of the invention in its simplest form, an input transformer 55 has its secondary connected through a. charging condenser 56 across a, rectifier 5l and a condenser 58, in parallel with which is a resistance 60 having a bias pass filter resistance I9 and condenser I8 are shown connected between` condenser 58 and output terminals 2.1 to prevent rapid voltage nuctuations from being impressed on the load whichl is generally the grid of a. vacuum tube. The

voltage discharge characteristics of the above circuit are illustrated in Fig. 2, wherein condenser 58 and resistance 60 may be given values so that the discharge rate' is as shown in curve I when no bias 6I is employed.4 With the bias 6l of a certain value the discharge characteristic is as shown in curve J. By using selected values of resistance 60 and bias i6l, any desired characteristic between curves I and J may be obtained.

shown in Although the use of the charging rectifier 56 normally prevents discharging beyond zero, rectier 51 is shunted across condenser 58 and through which the current from the bias source 6| can flow after the voltage on condenser 58 has reached zero, to prevent any possibility that a' small signal of peculiar wave form would permit the output of the circuit to go beyond zero or reverse, this being Vthe sole function of recti-` er 5l. Neither of the characteristics I or J Fig. 2 is particularly desirable since the voltage-at the cessation of a signal should decrease slowly for a short time interval to prevent utter and then'decrease rapidly to obtain the maximum noise reduction. The principles of operation shown 'in Figs. 1 and 2 are utilized Cin the following circuits to produce these desiravble results.

Referring now to Fig'. 3, showing another form of the invention, an alternating current signal is impressed upon primary of a transformer 8, secondary 1 of which is connected to a charging rectifier 9. The secondary 1 of transformer 6 is tapped, the voltage between points II landIZ after rectification by rectifier 9 being impressed across a condenser I5 and a resistance I6. The voltagegenerated across the entire secondary of transformer 6 between the points II and 2| after rectiiicaticn by rectifier 9 is impressed across a condenser 23 and a resistance 24 over a rectifier 25. 'I'he usual low pass filter is shownl byl con-v denser I8 and resistance'IS.

In the above circuit the resistance I6 is high compared with the resistance 24, so that condenser I5 discharging through resistance I6 will ydo so at a slowrate compared with the discharge rate of condenser 23 through resistance 24. To 'illustrate the rates of these discharges, reference is made to Fig; 4, wherein curve A represents the rate of voltage decrease across condenser I5,

while curve B represents the rate of voltage decrease across condenser 23, both of these curves being of the'exponential type. It will be noted that curve B is similar to curve I of Fig. 2. Neither of thevoltage characteristics A or Bis desirable for controlling a noise reduction element since curve A would permit too large an amount of transparent area to be present inthe iinabprint,

introducing noise, while curve B wouldintroduce nals. -As mentioned above, the voltage variation desired, is a slow decrease for -a short time interval after the signal ceases, and a rapid decrease thereafter. In Fig. 4'it willl be' observed that curve A provides the desired discharge rate immeshown in Fig. 1, the use of such a bias battery will produce a more rapid discharge of the condenser 23 as shown by curve C in Fig. 6, corresponding to curve J of Fig. 2, the complete discharge voltage characteristic being shown by the solid line curve in Fig. 7. The curve of Fig. 7 is made up of a section a'-b comparable to section a--b of Figs. 4 and 5 and a section a'c' comparable to section a-c of Fig. 6.

Referring now'to Fig. 8, the invention is shown embodied in a double discharge circuit such as disclosed andclaimed in E. W. Kellogg Patent No. 2,227,906 of'January '7, 1941 (see Fig. 6 of patent). In this arrangement the input transformer 30 is connected to rectiiiers 3l and 32 in parallel, rectifier 3l charging a condenser 34 and rectiiier 32 charging a condenser 35, the condensers being separated by a resistance 36 while the lter resistance I9 and condenser I8 are connected as shown in Fig. 1. In this arrangement it is' to be observed that a rectifler 31 is connected across condenser 34 in parallel with a series circuit comprising a resistance 38 and a bias source of potential 39 similar to the arrangement in Fig. 1.

To illustrate the voltage variation produced at the output terminals 21, reference is made to Figs. 9 to 12, inclusive, wherein Fig. 9 represents the discharge characteristic for'condenser 34 without the potentialsource 39 and is similar to curve I of Fig. 2, while the curve in Fig. 10 represents the voltage discharge characteristic for the condenser without the bias 39 inthe circuit. Because the condenser 35 cannot start to 'discharge A until the condenser 34 has discharged appreciably flutter during the recording of low frequency sigf diately after cessation of the signal, while curve B provides a more desirable closing rate vthereafter. 'I'his combination action'is shown,I by the full curve in Fig. 5 and isobtainable with the circuit shown as follows Since the condenser 23 is charged tc a higher voltage than condenser I5, preferably' 40 percent higher, as it is across portion II-2I of secondary 1, and condenser I5 is only across section II-I2, there will -be an appreciable time interval before the decreasing voltage across condenser 23 reaches voltage acrossv I5 and current can now flow through rectifier 25. The voltage at terminals 21' will now decrease rapidly substantially along the characteristic curve B of condenser 23 as shown between points a and c in Figs. 4 and 5, the complete discharge voltage characteristic being shown by the full line curve of Fig. 5. l

A more'rapid closing time than that between points a and c in Fig.`5 may be accomplished -by placing a battery 0r similar source of energy in series with resistance 24 across condenser 23. As

and then it must discharge through resistance 36, the curve of Fig. 10 shows a desirable slow starting discharge rate. However, the rate of discharge of condenser 35 thereafter is too slow to be desirable. To increase this later rate of discharge the biassource 39 is employed.

To illustrate the action of the circuit in Fig. 8 with the bias 39, reference is made to Figs. 11 and 12, wherein the curve shown in Fig. llshows a rapid discharge rate -for the condenser 34 with the bias battery 39 in series with resistance 38 and is `similar 'to curve J of Fig. 2. Since this discharge ratte is now rapid, the discharge of condenser 35 will substantially follow, and the voltage decrease of condenser 35/wi1l beas shown by Fig. 12. However, there is a possibility'that with the use of the bias 39 small signals of peculiar wave form may cause the discharge to overshoot and reverse as pointed out above in connection with Fig. 1.

Such action is prevented by the rectier 31 which will pass current with a reverse voltage and thus short-circuit the condenser 34, preventing any reversecharge being placedV thereon.

` Referring now to Fig. 13, a combination of the system of Figs. -3 and 8 is illustrated, wherein the input transformer 40 has its secondary tapped to provide sections 41-42 and 4I-43. v Section 4I-42 charges through a rectifier '45, va condenserv 41 shnted by a resistance 5I). The larger section 4|-'-43 of transformer 40 charges a condenser 48 through rectifier 49 and a condenser 5I through rectier 52, a resistance 53 separating condensers 48 and 5I. Connected ,acr ss condenser 43 is a resistance 54.

The operation of the above descri ed circuit is illustrated in the graphs shown in Figs, 14 to 1'1, inclusive, which will now be referred to. The

condenser 41 will discharge through the resist# ance 5I] at a slow rate, as shown by the curve D of Fig. 14. this curve being. similar to curve A of Fig. 4.V Condenser 48 will discharge more rapidly through low resistance 54 as shown by curve E in Fig. 14 which is similar to curve B of Fig. 4 or curve I of Fig. 2. The condenser will discharge similarly to ,condenser 35 of Fig. 8, as shown by curve F in Fig. 14, and in the curve of Fig. 10. In Fig. 15 the resulting voltage discharge across condenser 41 is illustrated, this full-line curve being a combination of curves D and F of Fig. 14. This characteristic is produced similarly to the manner of obtaining the curve of Fig. 5. Ihat is, the condenser 41 and resistance 50 control the load voltage until the voltage across condenser 5| equals that of condenser 41, at which time the discharge of condenser 41 is through resistance 50 and the resistances 53 and.54 in parallel with resistance 50. The discharge volt-j age will-then substantially follow curve F of Fig. 14.

As described in connection with Figs. 1, 3 and 8,

if a bias voltage is used as shown in Figs. 1 and 8,

\ the condenser 41 and resistance 50 combination \will still have the same normal rate oi' discharge as shown by curve D in Fig. 16, while condenser 48 will have a more rapid discharge rate a-s shown by curve G-and condenser 35 will have a discharge'ncharacteristic as shown by curve H. The combination of curves D and H will provide the final discharge voltage variation across terminals 21 as shown by the full-line curve in Fig. 1'?.

From the above description it is to be observed that by use of the abov -described circuits any desired closing characteristic is obtainable, this characteristic being of a form wherein over a short period after the cessation of a signal, the voltage decrease upon the control tube is at a slow rate, followed by a more rapiddischarge rate to insure maximum noise reduction Without iiutter.

I claim asmy invention: 1. In a noise reduction circuit, the combination of a source of alternating currents, means for rectifyingv said alternating currents, a plurality of condensers charged by said rectified currents, means for individually controlling the. rate of dislcharge of each of said condensers to producey a voltage variation which is the substantial) combination of said individual discharge rates, and means for biasing one of said individual discharge. rate controlling means for increasing the rate of discharge of one of said condensers the normal rate of discharge thereof.

2. In a noise 'reduction circuit, the combination of a source of alternating currents, means for rectiiyingY said alternating currents, a plurality of condensers charged by said rectiiied currentsfmeans for individually controlling the lrate of'discharge of each of duce a voltage variation which is' the substantial means for charging said condensers to diii'erent values, and a source of biasing potential for one of said individual discharge rate controlling means for increasing the normal rate of discharge thereof.

3. A noise reduction circuit comprising an input transformer having a tapped secondary winding, a charging rectiiier, a plurality of condensers, means connecting one of said condensers to a position on said secondary to obtain a certain charge on said-condenser, means connecting another of said condensers to another position on said secondary to obtain a different charge thereon, means shunting each of said condensers to obtain different discharge rates therefor, and means for isolating the discharge of one of said condensers from aiecting the discharge of said other condenser until the charges on each of said condensers are equal.

4. A noise reduction circuit in accordance with claim 3 in which said last-mentioned means comprises a second rectiiier in series with said condensers to provide a single direction discharge path for said condensers,

5. -A noise reduction circuit in accordance with claim 3 in which means are provided in the discharge path of said higher charged condenser to increase the discharge rate thereof.

6. A noise reduction circuit comprising a transformer, a charging rectiiier, a condenser adapted to be charged by rectied currents from said rectiiier,` a resistance discharge path including a resistor shunting said condenser, and a source oi bias potential in said resistive discharge path for increasing the rate of discharge of said condenser.

7. A noise reduction circuit in accordance with claim 6 in which a rectier is providedain shunt to said condenser and resistive discharge path to prevent a reverse charge on said condenser.

to increase said condensers to pro- Y 8. A noise reduction circuit comprising a transformer, a plurality of charging rectiiiers, a-re' `spective plurality of condensers, each charged by an individual rectifier, a resistive discharge path including a source of bias potential for one of said condensers, and another condenser having a resistive discharge path for controlling the output voltage 'of said circuit, the discharge rate of said last-mentioned condenser being controlled by the rate of discharge of said mst-mentioned condenser over a last portiony otits discharging time interval.

9. A noise reduction circuit in accordance with to said first-mentioned condenser to prevent a reverse charge on said condenser.

RBERT- L. anims. 

